Continuously variable transmissions are employed in powertrains between an engine and final drive gearing to improve the operating range of the engine. Also these transmissions permit the engine to operate at the most efficient fuel consumption. Once the engine operating speed is achieved, the output speed of the vehicle can be further adjusted by changing the ratio in the transmission. With a continuously variable transmission, small but effective ratio changes can be attained. This is in contrast to a fixed gear ratio unit where any ratio changes are step values. This requires the engine to operate through the full speed range for each transmission ratio.
One drawback of the continuously variable transmission, as compared with a planetary gear arrangement, is that a true neutral condition cannot be achieved. This requires that a starting device, such as a clutch or fluid coupling, also be incorporated into the powertrain. This drawback is at least partially overcome by incorporating a planetary gear set between the engine and the transmission. The planetary gear set will permit a geared neutral condition under which the variable transmission ratio is set to take advantage of the planetary gear ratio when the variable members of the transmission are connected to respective ones of the planetary transmission and the transmission output shaft is connected to the third planetary member.
Geared neutral transmissions include a continuously variable unit (CVU) combined with a planetary gear unit. The CVU most widely proposed for such transmissions is of the belt and pulley type. One such geared neutral belt and pulley variable transmission is disclosed in U.S. Pat. No. 4,644,820 issued to Macey et al on Feb. 24, 1987. The planetary gear unit is effectively connected between the input and output components of the CVU to permit the input and output components to rotate while the output of the planetary gear unit is stationary. These transmissions provide a power split in the low or launch range, immediately following neutral, however when these transmissions have a second range, all of the power from the engine passes through the CVU which reduces the overall efficiency and power capabilities of the transmissions.